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PubMed2006

Time and frequency domain responses of the mechanomyogram and electromyogram during isometric ramp contractions: a comparison of the short-time Fourier and continuous wavelet transforms.

The purposes of this study were to examine the mechanomyographic (MMG) and electromyographic (EMG) time and frequency domain responses of the vastus lateralis (VL) and rectus femoris (RF) muscles during isometric ramp contractions and compare the time-frequency of the MMG and EMG signals generated by the short-time Fourier transform (STFT) and continuous wavelet transform (CWT). Nineteen healthy subjects (mean+/-SD age=24+/-4 years) performed two isometric maximal voluntary contractions (MVCs) before and after completing 2-3, 6-s isometric ramp contractions from 5% to 100% MVC with the right leg extensors. MMG and surface EMG signals were recorded from the VL and RF muscles. Time domains were represented as root mean squared amplitude values, and time-frequency representations were generated using the STFT and CWT. Polynomial regression analyses indicated cubic increases in MMG amplitude, MMG frequency, and EMG frequency, whereas EMG amplitude increased quadratically. From 5% to 24-28% MVC, MMG amplitude remained stable while MMG frequency increased. From 24-28% to 76-78% MVC, MMG amplitude increased rapidly while MMG frequency plateaued. From 76-78% to 100% MVC, MMG amplitude plateaued (VL) or decreased (RF) while MMG frequency increased. EMG amplitude increased while EMG frequency changed only marginally across the force spectrum with no clear deflection points. Overall, these findings suggested that MMG may offer more unique information regarding the interactions between motor unit recruitment and firing rate that control muscle force production during ramp contractions than traditional surface EMG. In addition, although the STFT frequency patterns were more pronounced than the CWT, both algorithms produced similar time-frequency representations for tracking changes in MMG or EMG frequency.

Adult↗

PubMed1984

A study of fatigue during repetitive static work performed in two different segmental positions.

Sixteen subjects, nine women and seven men, aged between 19 and 35 years, performed three series of isometric contractions of the flexor muscles of the forearm at 40% MVC. Each series consisted of four isometric contractions sustained until exhaustion, and separated from each other by an incomplete rest interval of 5 min. The position of the arm (either the lower or the upper position) was variable according to the series. Under these conditions, study of heart rate (HR), systolic blood pressure (SBP) and limit-time showed that: HR and SBP were not affected by the arm position even when fatigue was important. For a given load with a progressive appearance of fatigue, the limit-time in female subjects was higher. In both groups, in spite of the fatigue appearing after several isometric contractions performed in the upper position, movement of the arm to the lower position was sufficient to significantly increase the time-limit of another isometric contraction at 40% MVC without concomitant increase in cardiac work.

Adult↗

PubMed1994

Eccentric contraction-induced injury of mouse soleus muscle: effect of varying [Ca2+]o.

The objective of this study was to determine the effect of varying extracellular Ca2+ concentration ([Ca2+]o) on eccentric contraction-induced muscle injury. Isolated mouse soleus muscles (n = 64) performed either 20 eccentric or 20 isometric contractions over a 40-min period in a Krebs buffer containing 0.5, 1.25, or 5.0 mM Ca2+. Measurements of contractile function and lactate dehydrogenase accumulation in the buffer were then made every 15 min for 2 h. Prostaglandin E2, leukotriene B4, and tyrosine accumulation in the incubation medium and total muscle [Ca2+] were measured at the end of the experiment. Reductions in maximal isometric tetanic force for muscles immediately after performance of 20 eccentric and 20 isometric contractions were 21.1 +/- 1.4 and 1.2 +/- 0.7%, respectively. Total muscle [Ca2+] was 28-37% higher in muscles that performed eccentric contractions than in those that performed isometric contractions. However, estimates made with a confocal laser scanning microscope and fluo 3 do not indicate that there was a difference in free cytosolic [Ca2+] between fibers from injured and control muscles. Also, leukotriene B4, prostaglandin E2, and tyrosine accumulation in the buffer from muscles that performed eccentric contractions was not elevated over that from muscles that performed isometric contractions. Furthermore, lactate dehydrogenase accumulation and reductions of contractile function over the 2-h incubation period were not enhanced by higher [Ca2+]o or influenced by the type of contraction. These findings suggest that muscles that were injured by eccentric contractions were able to buffer the increased influx of extracellular Ca2+, maintain a normal free cytosolic [Ca2+], and avoid activation of Ca(2+)-sensitive degradative pathways.

Aniline Compounds↗

PubMed1986

Age-related differences in effect of exercise training on cardiac muscle function in rats.

To test the hypothesis that age may alter the effect of exercise training on myocardial function, we trained young adult (4-5 mo at study initiation) and old (21 mo at initiation) male Fischer 344 rats to run on a rodent treadmill 5 days/wk for 4.5 mo and compared their myocardial function with that of age-matched sedentary controls. Right ventricular papillary muscles were isolated from the young adult and aged animals and stimulated at the length at which developed isometric tension was maximal to contract isometrically at 12/min, bathed in oxygenated modified Krebs solution, and studied at 28 degrees, 32 degrees, 35 degrees, and 38 degrees C. In the absence of cardiac hypertrophy, exercise training significantly reversed the age-related prolongation of isometric contraction duration in the senescent rat (P less than 0.001). However, exercise training prolonged contraction duration in the young adult rat (P less than 0.05). The relaxation phase was more sensitive than the contraction phase to changes in temperature, especially in the aged rat (P less than 0.02). The ratio of developed tension to time-to-peak tension of the isometric twitch was not affected by exercise training. Because exercise training affects cardiac function in opposite directions from young adulthood to senescence, caution is advised in extrapolating results from exercise studies in young adult rats to populations of older animals.

Aging↗

PubMed1976

Cycling aggregation patterns of cytoplasmic F-actin coordinated with oscillating tension force generation.

Isometric contracting protoplasmic veins of Physarum polycephalum show cycling patterns of cytoplasmic F-actin, dependent on their oscillating contraction behaviour (minute rhythms). The process of fibrillogenesis represents a parallel arrangement of F-actin chains ("plasma filaments, microfilaments") during the isometric contraction phase. A part of the results of the present work corroborates previous results on stretch-activated veins which showed that the fibrillar form of F-actin reflects the isometric contracted state. During isometric relaxation phase, a disaggregation of the fibrillar pattern takes place and is accompanied by a deparallelisation of F-actin chains. Therefore, the isometric relaxed state of cytoplasmic actomyosin is non-fibrillar in nature. Thus, the morphologically detectable fibrillar form of cytoplasmic actomyosin, according to physiological interpretation, is solely representative of the isometric contracted state. The question whether assembly-disassembly processes, e.g. G equilibrium F-actin-transformation, play a role in the contraction-relaxation cycle is discussed.

Actins↗

PubMed1991

An equilibrium-point model for fast, single-joint movement: II. Similarity of single-joint isometric and isotonic descending commands.

The model for isotonic movements introduced in the preceding article in this issue is used to account for isometric contractions. Isotonic movements and isometric contractions are analyzed as consequences of one motor program acting under different peripheral conditions. Differences in isotonic and isometric EMG patterns are analyzed theoretically. Computer simulation of the EMG patterns was performed both with and without the inclusion of possible effects of reciprocal inhibition. A series of experiments was performed to test the model. The subjects made fast isotonic movements that were unexpectedly blocked at the very beginning in some of the trials. The observed differences in the EMG patterns between blocked and unblocked trials corresponded to the model's predictions. The results suggest that these differences are due to the action of a tonic stretch reflex rather than to preprogrammed reactions. The experimental and simulation findings, and also the data from the literature, are discussed in the framework of the model and the dual-strategy hypothesis. They support the hypothesis that the motor control system uses one of a few standardized subprograms, specifying a small number of parameters to match a specific task.

Journal Article↗

PubMed1982

Isometric fatigue induced by different levels of rhythmic exercise.

Three subjects were trained in leg extensor isometric contractions and in cycling. They then cycled for three consecutive bouts, each of 2.75 min at a constant level of VO2, from 20 to 80% VO2 max. Fifteen seconds after each bout of cycling the subjects exerted an isometric contraction of the right leg at 40% of the maximum voluntary contraction. In each experiment, the duration of the three successive isometric contractions decreased as in hand-grip contractions. There was also a linear reduction in isometric endurance as the severity of the preceding rhythmic exercise increased. In other experiments, after three bouts of rhythmic exercise at 20% VO2 max (each followed by a fatiguing contraction at 40% MVC), further bouts of cycling at increasing levels of severity up to 60% VO2 max resulted in a linear fall in isometric endurance which could be reversed by interposing a lighter level of cycling. The heart rates during these experiments showed a steady increase during the isometric exercise, to about 150 beats X min-1, as the bouts of preceding rhythmic exercise became progressively more severe. The isometric contractions had little influence on the heart rate during cycling. But the rhythmic cycling exercise markedly increased the heart rate achieved at the end of the isometric contractions while decreasing the increment in heart rate during the contraction.

Adult↗

PubMed2004

H-reflex and reciprocal Ia inhibition after fatiguing isometric voluntary contraction in soleus muscle.

This study investigated the H-reflex and reciprocal Ia inhibition during fatigue in the human soleus muscle. Ten healthy subjects participated in this study, and performed intermittent isometric voluntary contraction of the ankle plantarflexion at 50% MVC as the fatiguing task. Reciprocal Ia inhibition was evaluated by the degree of H-reflex amplitude depression in the soleus muscle by the test stimulus following conditioning stimulus to the common peroneal nerve. The difference in H-reflex amplitude between before and after fatiguing task was also checked. There was no significant difference in the degree of H-reflex amplitude depression, although the H-reflex amplitude significantly decreased after the fatiguing task (p < 0.01). From the results of this study, it was considered that the decrease in H-reflex amplitude was caused by descending inhibitory input from the supraspinal to alpha-motoneuron, and the excitability of the Ia inhibitory interneuron was not involved. It was suggested that the function of reciprocal Ia inhibition was difficult to modulate during fatigue caused by isometric voluntary contraction in this study.

Adult↗

PubMed2000

Expression of c-fos-like immunoreactivity in the feline brainstem in response to isometric muscle contraction and baroreceptor reflex changes in arterial pressure.

This study compared whether activation of muscle ergoreceptor afferents caused by isometric muscle contraction, activation of baroreceptor afferents induced by i.v. infusion of phenylephrine, or baroreceptor afferent inactivation, caused by carotid artery occlusion, elicit similar patterns of c-Fos induction in brainstem areas. Adult cats were anesthetized with alpha-chloralose, and in each case, the experimental intervention caused an increase in the arterial blood pressure. There were two sets of control experiments: in both, animals underwent the same surgical procedures but then either remained at rest for the entire study, or the tibial nerve was stimulated, as in the contraction group, following muscle paralysis with tubocurarine. Following the procedures, animals rested for 90 min to allow neuronal expression of c-Fos. Control cats showed very little c-Fos immunoreactivity (c-Fos-ir) in the brainstem. Muscle contraction induced c-Fos-ir expression mainly in the nucleus tractus solitarius, lateral reticular nucleus, lateral tegmental field, vestibular nucleus, subretrofacial nucleus, spinal trigeminal tract and in a lateral region of the periaqueductal grey (P 0.5-1.0). The majority of the c-Fos-ir was found in brainstem areas contralateral to the contracted muscle. In addition, muscle contraction induced c-Fos-ir in the dorsal horns of spinal segments L6-S1 on the ipsilateral side of the spinal cord. Phenylephrine infusion caused c-Fos-ir expression in the nucleus tractus solitarius, spinal trigeminal tract, solitary tract, and dorsal motor nucleus of the vagus. No c-Fos-ir was apparent in the periaqueductal grey. Carotid occlusions induced c-Fos-ir expression in the area postrema, nucleus tractus solitarius, solitary tract, and spinal trigeminal tract. Expression was bilateral. Areas that exhibited c-Fos-ir correspond to sites previously reported to release various neuropeptides in response to muscle contraction or carotid occlusions. These results indicate that the exercise pressor reflex and baroreflex activate similar, but not completely identical, sites in the brainstem.

Afferent Pathways↗

PubMed2004

Neuromuscular efficiency of the triceps surae in induced and voluntary contractions: morning and evening evaluations.

Variations in force and electromyographic (EMG) activities of skeletal muscles with the time-of-day have been previously described, but not for a postural muscle, submitted to daily postural and locomotor tasks. In this article, mechanical performances, EMGs, and the ratio between these parameters, i.e., the neuromuscular efficiency (NME), were measured on the triceps surae (TS) of eight subjects, two times each day, at 6:00 and 18:00 h. NME was evaluated under different experimental conditions (electrically induced contractions, reflex contractions, maximal and submaximal voluntary isometric contractions, and during a natural movement, a drop jump) to determine whether mechanisms, peripheral or central in origin, were responsible for the eventual changes in NME with time-of-day. To calculate NME in induced conditions (NMEind), a supramaximal electrical stimulus was applied to the tibial nerve, and the maximal M wave of TS (TS Mmax) and the amplitude of the twitch tension (PtMmax) in response to this electrical stimulation were quantified. TS Mmax was significantly lower in the evening (mean gain value -10.7 +/- 5.5%, p < 0.05), whereas PtMmax was not significantly modified. NMEind (PtMmax/TS Mmax) was significantly higher in the evening (mean gain of 17.6 +/- 5.8%, p < 0.05), and this increase was necessarily peripheral in origin. Secondly, maximal tendon taps were applied to the Achilles tendon in order to quantify at the two times-of-day the reflexes in response to a mechanical stimulus. The maximal reflex, TS Tmax/Mmax (%), the peak amplitude of the twitch tension associated to this tendon jerk (PtTmax), and the corresponding NME (NMEreflex = PtTmax/TS Tmax/Mmax) were not affected by time-of-day, indicating that reflex excitability did not present daytime variations when tested under these conditions. Voluntary isometric contractions were required under maximal (MVC) and submaximal (25% MVC) conditions, and the corresponding torques and TS EMG were measured. MVC was higher in the evening (mean gain: 8.6 +/- 2.7%, p < 0.05) and TS EMGmax (normalized with regard to TS Mmax) also increased in the evening but not significantly; thus, NMEMvc was not modified. At 25% of MVC, TS EMG was significantly higher in the evening (mean gain of 23 +/- 13.9%, p <0.05) and a trend for a lower NME25%MVC in the evening was observed, a result probably representative of a higher muscle fatigue state in the evening. Finally, to test the muscle capacities during a natural task, a NME index was calculated during a drop jump (DJ). The NMEDJ was defined as the ratio between jump height and mean amplitude of TS EMG (% of TS Mmax) between the drop and the jump. Both jump height and NMEDJ were significantly higher in the evening (mean gains of 10.9 +/- 4.5% and 15.7 +/- 7.4%, respectively, p <0.05). In conclusion, daytime changes in the efficiency of postural muscles seem to depend on both peripheral and central mechanisms. According to the experimental conditions, NME of the postural muscle could increase, remain constant, or even decrease in the evening, and this result may reflect reverse effects of better contractile capacities and higher fatigue state.

Adolescent↗

PubMed1994

Q angle: effects of isometric quadriceps contraction and body position.

The quadriceps femoral angle (Q angle) has been linked with several knee disorders, but Q-angle measurement procedures have not been standardized. The purpose of this study was to examine the effects of isometric quadriceps contraction in the standing and supine positions on the Q angle. The Q angles of the right knees of 30 men and 30 women were measured goniometrically during four test conditions: 1) quadriceps relaxed in standing, 2) quadriceps contracted in standing, 3) quadriceps relaxed in supine, and 4) quadriceps contracted in supine. The pelvic widths were also measured as the distance between the anterior superior iliac spines in standing and supine. A two-way multivariate analysis of variance showed a significant difference between genders (p < 0.001) and among the four test conditions (p < 0.001) (N = 60). Separate one-way analyses of variance (ANOVAs) with two grouping factors (contraction and position) showed significant effects of contraction (p < 0.001) for both men and women, but no significant effect of positions. A dependent t-test showed that the pelvic width differed between standing and supine for both men and women (p < 0.001). The results showed that isometric quadriceps contraction affects the Q angle with the subjects standing or supine, even though the pelvic width differed significantly between the two positions. Clinicians and researchers should consider the results of this study in order to standardize procedures for measuring the Q angle.

Adult↗

PubMed2000

Electromyographic analysis of exercise resulting in symptoms of muscle damage.

Surface electromyographic (EMG) signals were recorded from the hamstring muscles during six sets of submaximal isokinetic (2.6 rad x s(-1)) eccentric (11 men, 9 women) or concentric (6 men, 4 women) contractions. The EMG per unit torque increased during eccentric (P < 0.01) but not during concentric exercise. Similarly, the median frequency increased during eccentric (P < 0.01) but not during concentric exercise. The EMG per unit torque was lower for submaximal eccentric than maximum isometric contractions (P < 0.001), and lower for submaximal concentric than maximum isometric contractions (P < 0.01). The EMG per unit torque was lower for eccentric than concentric contractions (P < 0.05). The median frequency was higher for submaximal eccentric than maximum isometric contractions (P < 0.001); it was similar, however, between submaximal concentric and maximum isometric contractions (P = 0.07). Eccentric exercise resulted in significant isometric strength loss (P < 0.01), pain (P < 0.01) and muscle tenderness (P < 0.05). The greatest strength loss was seen 1 day after eccentric exercise, while the most severe pain and muscle tenderness occurred 2 days after eccentric exercise. A lower EMG per unit torque is consistent with the selective recruitment of a small number of motor units during eccentric exercise. A higher median frequency during eccentric contractions may be explained by selective recruitment of fast-twitch motor units. The present results are consistent with the theory that muscle damage results from excessive stress on a small number of active fibres during eccentric contractions.

Adult↗

PubMed2000

Motor-unit synchronization is not responsible for larger motor-unit forces in old adults.

Motor-unit synchronization, which is a measure of the near simultaneous discharge of action potentials by motor units, has the potential to influence spike-triggered average force and the steadiness of a low-force isometric contraction. The purpose of the study was to estimate the contribution of motor-unit synchronization to the larger spike-triggered average forces and the decreased steadiness exhibited by old adults. Eleven young (age 19-30 yr) and 14 old (age 63-81 yr) adults participated in the study. Motor-unit activity was recorded with two fine-wire intramuscular electrodes in the first dorsal interosseus muscle during isometric contractions that caused the index finger to exert an abduction force. In a separate session, steadiness measurements were obtained during constant-force isometric contractions at target forces of 2.5, 5, 7. 5, and 10% of the maximum voluntary contraction (MVC) force. Mean (+/-SD) motor-unit forces measured by spike-triggered averaging were larger in old (15.5 +/- 12.1 mN) compared with young (7.3 +/- 5.7 mN) adults, and the differences were more pronounced between young (8.7 +/- 6.4 mN) and old (19.9 +/- 12.2 mN) men. Furthermore, the old adults had a reduced ability to maintain a steady force during an isometric contraction, particularly at low target forces (2.5 and 5% MVC). Mean (+/-SD) motor-unit synchronization, expressed as the frequency of extra synchronous discharges above chance in the cross-correlogram, was similar in young [0.66 +/- 0.4 impulses/s (imp/s); range, 0.35-1.51 imp/s; 53 pairs) and old adults (0.72 +/- 0.5 imp/s; range, 0.27-1.38 imp/s; 56 pairs). The duration of synchronous peaks in the cross-correlogram was similar for each group (approximately 16 ms). These data suggest that motor-unit synchronization is not responsible for larger spike-triggered average forces in old adults and that motor-unit synchronization does not contribute to the decreased steadiness of low-force isometric contractions observed in old adults.

Action Potentials↗

PubMed2006

Partition between volitional and induced forces in electrically augmented dynamic isometric muscle contractions.

Augmentation of force in partially deficient muscles can be achieved by combining electrical stimulation (ES) with their volitional activation (hybrid activation). However, while the overall torque results from the combination of the volitional and the electrically-induced torque components, the exact share between these components is not known. In a previous work, we described a method to resolve the share between the torque components under isometric static contractions. In this work, we extend our analysis to the case of isometric dynamic contractions. Five healthy subjects were instructed to contract their Tibialis Anterior (TA) muscles according to a typical gait-like dynamic torque pattern, that was visually displayed to them, while monitoring their actual ankle torque and TA electromyography (EMG). These experiments were done with and without augmented activation by means of ES. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG signal processing and on precalibration of the dynamic system of the volitional torque versus EMG. The results indicated the quantitative relations between decrease in the volitional torque and the required increase in ES enhancement. The developed method also demonstrated what ES intensity profile is necessary to produce a desired overall torque output. This provides the means for designing an adaptive rehabilitation device for the hybrid activation of deficient muscles.

Adult↗

PubMed1986

Increase in electromyogram low-frequency power in nonfatigued contracting skeletal muscle.

We studied the relationship between changing elbow joint angle and the power spectral density of the biceps brachii muscle electromyogram (EMG) during submaximal isometric contractions. For this purpose, we recorded the EMG of the biceps brachii muscle with surface electrodes in 13 subjects. Each subject held a 2.8-kg weight and contracted the biceps isometrically for 30 s at one of two lengths. The length of the muscle was changed by flexing the forearm toward the upper arm to form an angle of 135 degrees (L1) or 45 degrees (L2). We found that the mean centroid frequency (fc) of the EMG power spectral density was 26% lower at L1 than at L2 (P less than 0.01). For each subject there was no significant change in fc during the isometric contraction at either angle. In addition, in nine subjects who sustained fatiguing contractions of the biceps with a 6-kg load, fc decreased by 15% (P less than 0.025). These data suggest that a change in the length at which a muscle contracts isometrically can alter or induce indirectly an alteration in the frequency content of its EMG. This finding may have important implications for the assessment of respiratory muscle EMG especially during loaded breathing.

Adult↗

PubMed1978

Recruitment order of human spindle endings in isometric voluntary contractions.

1. The responses of twenty-two spindle endings in the anterior tibial and toe extensor muscles of human subjects were studied during isometric voluntary contractions of the receptor-bearing muscle with the ankle joint fixed in 25 degrees plantar flexion.2. The discharge of eighteen endings accelerated in voluntary contractions when the contraction strength exceeded a threshold level which differed for different endings but was reproducible for the same ending.3. With contractions of slow onset the latency to spindle acceleration varied with the speed of onset of the contraction. Endings with a background discharge were often unloaded by contractions until the contraction strength exceeded the threshold for activation of the ending.4. No correlation was found between the sensitivity of a spindle to external length changes and its ease of activation in a voluntary contraction. For two spindle endings with a background discharge there was no change in either discharge frequency or the regularity of spindle discharge during contractions which were below the threshold for activation of the endings. It is concluded that the threshold for activation of a spindle ending in an isometric voluntary contraction is determined by its fusimotor innervation, and that fusimotor neurones probably have a recruitment order, much as do skeletomotor neurones.5. Once activated, the discharge of spindle endings fluctuated with changes in skeletomotor activity but the relationship for some endings contained non-linearities. Such non-linearities were not as apparent in multi-unit recordings from a number of spindle endings in the contracting muscle. It is concluded that the fusimotor drive to a muscle is proportional to the skeletomotor drive to the muscle, and that skeletomotor and fusimotor neurones are subjected to similar if not identical descending command signals. From the work of Evarts (1968), it seems likely that these command signals are related more to desired muscle force than to desired muscle length.

Action Potentials↗

PubMed2004

Matrix metalloproteinase-specific inhibition of Ca2+ entry mechanisms of vascular contraction.

OBJECTIVE: Abdominal aortic aneurysm (AAA) is a common disease with as yet unclear cause. Increased matrix metalloproteinase (MMP) levels in the plasma and aorta are a consistent finding in AAA. Although the role of MMPs in AAA has largely been attributed to degradation of the extracellular matrix proteins, the effects of MMPs on the mechanisms of aortic contraction are unclear. The purpose of this study was to test the hypothesis that MMPs promote aortic dilation by inhibiting the Ca2+ mobilization mechanisms of smooth muscle contraction. METHODS: Isometric contraction and 45Ca2+ influx were measured in aortic strips isolated from male Sprague-Dawley rats treated or not treated with MMP-2 and MMP-9. RESULTS: In normal Krebs solution (2.5 mmol/L Ca2+ ) phenylephrine (10-5 mol/L) caused contraction of the aortic strips, which was significantly inhibited (P < .05) by MMP-2 (maximum, 48.9% +/- 5.0%) and to a greater extent by MMP-9 (maximum, 69.8% +/- 6.2%). The MMP-induced inhibition of phenylephrine contraction depended on concentration and time. The inhibitory effects of MMPs on phenylephrine contraction were reversible. In Ca2+ -free (2 mmol/L ethylene glycol bis[beta-aminoethyl ether]-N,N,N',N'-tetraacetic acid) Krebs solution phenylephrine caused a small contraction that was not inhibited by MMP-2 or MMP-9, which suggests that MMPs do not inhibit Ca2+ release from the intracellular stores. Membrane depolarization with 96 mmol/L of potassium chloride, which stimulates Ca2+ entry from the extracellular space, caused a time-dependent and reversible contraction, which was inhibited by MMP-2 and MMP-9. Histologic studies of MMP-treated tissues stained with hematoxylin-eosin or Verhoeff stain for elastin confirmed the absence of degradation of the extracellular matrix. MMP-2 and MMP-9 also caused significant inhibition of 45Ca2+ influx induced by phenylephrine and potassium chloride. CONCLUSIONS: These data suggest that MMP-2 and MMP-9 promote aortic dilation by inhibiting the Ca2+ entry mechanism of vascular smooth muscle contraction. CLINICAL RELEVANCE: Abdominal aortic aneurysm (AAA) is a slow and progressive disease. The late stages of AAA are characterized by degenerative changes in the extracellular matrix and smooth muscle components of the aortic wall. The present study describes novel inhibitory effects of matrix metalloproteinase (MMP) on the Ca2+ entry mechanisms of aortic smooth muscle contraction, even in the absence of extracellular matrix degradation. The MMP-induced inhibition of aortic contraction may further explain the role of increased MMP activity particularly during the early development of AAA. Chronic exposure to MMPs may lead to protracted inhibition of aortic contraction, progressive aortic dilation, and aneurysm formation. MMP-9 is a more potent inhibitor of aortic contraction than MMP-2, consistent with a more dominant role in AAA. Restoration and preservation of smooth muscle contractile function by specific inhibitors of MMPs may represent a new strategy in preventing the progression of small AAA.

Animals↗

PubMed1993

A single order-disorder transition generates tension during the Huxley-Simmons phase 2 in muscle.

Increasing temperature was used to progressively interconvert non-force-generating into force-generating states in skinned rabbit psoas muscle fibers contracting isometrically. Laser temperature-jump and length-jump experiments were used to characterize tension generation in the time domain of the Huxley-Simmons phase 2. In our experiments, phase 2 is subdivisible into two kinetic steps each with quite different physical properties. The fast kinetic component has rate constant of 950 s-1 at 1 degrees C and a Q10 of approximately 1.2. Its rate is tension insensitive and its normalized amplitude declines with rising temperature--behavior that closely parallels the instantaneous stiffness of the cross-bridge. It is likely that this kinetic step is a manifestation of a damped elastic element/s in the fiber. The slow component of phase 2 is temperature-dependent with a Q10 of approximately 3.0. Its rate is sensitive to tension. Unlike the fast component, its amplitude remains in fixed proportion to isometric tension at different temperatures indicating direct participation in tension generation. Similar T-jump studies on frog fibers are also included. The combined results (frog and rabbit) suggest that tension generation occurs in a single endothermic (entropy driven) step in phase 2.

Animals↗